Workpiece holders, or workholders, have been used for decades and are useful for holding solid and annular workpieces for subsequent machining or grinding. A first type of hydraulic workholder includes a main body, a circumferentially continuous steel sleeve carried on the body to grip a workpiece, and a hydraulic fluid chamber defined between the body and sleeve and sealed by rubber O-rings therebetween. A second type of hydraulic workholder includes a main body, a slotted steel sleeve or collet carried by the main body, a relatively hard (85 durometer Shore D scale) plastic bladder carried between the main body and the collet, and a hydraulic fluid chamber defined between the body and bladder and sealed by soft (70 durometer Shore A scale hardness) rubber O-rings therebetween.
With both workholder types, hydraulic fluid in the chamber is pressurized to radially displace the sleeve into gripping engagement with a workpiece. For example, to achieve 0.001″ expansion of a typical one-inch diameter sleeve the first type requires application of about 6,000 PSI and the second type requires about 200 PSI of fluid pressure. And, to achieve 0.020″ expansion with the second type requires about 2,000 PSI of fluid pressure. In extreme operating environments, such high pressures require concomitant energy consumption, may result in workholder leakage, and/or may cause the hard plastic bladder to extrude through slots in the collet, thereby permanently damaging the workholder.
One recent approach to avoid such high pressure damage includes using a thin metal baffle interposed between the bladder and the collet to prevent the bladder from being extruded through the collet. Even though this approach is a relatively low cost solution, it does not eliminate the high pressure conditions.
Another recent solution uses a bladder cartridge, which includes a U-shaped bladder carried on a corresponding T-shaped retainer, and axial sealing components to axially compress the bladder to the retainer. The axial sealing components are axially fastened to the retainer or threaded to each other to compress axial sealing protuberances on the bladder under extremely high forces to prevent hydraulic fluid from escaping a radial fluid chamber between the bladder and the retainer. Although this approach results in somewhat lower fluid pressures, it requires many additional parts of precision manufacture.
A further recent solution includes use of a multi-piece bladder including a 70 durometer (Shore A scale hardness) plastic center section fused between 90 durometer (Shore D scale) plastic end sections. The multi-piece bladder has a wall thickness of about 0.250″ to 0.500″ for a corresponding 1″ in outside diameter. The relatively hard end sections sealingly engage much softer 70 durometer (Shore A scale hardness) O-rings between the bladder and a main body in order to seal a fluid chamber therebetween. While this approach also results in lower fluid pressures, it too requires many additional parts of precision manufacture.
In summary, the art of workholding has used older high pressure workholders with steel or hard plastic bladders, or newer workholders employing more complex configurations such as high compression axial sealing arrangements or thick and hard multi-piece bladder materials.